Abstract: A method of injecting liquid using an injection device, the injection device includes an infrared transceiver, which transmits infrared lights, and receives reflected infrared lights that are reflected by an object. A nozzle, and a control unit further are included in the injection device. The control unit is in communication with the infrared transceiver and the nozzle through a data bus. Once the control unit detects a size of the object according to a number of the reflected infrared lights that are received by the infrared transceiver, the control unit controls the nozzle injecting liquid with a predetermined volume according to the size of the object.